1. Field of the Invention
This invention relates to a signal processing apparatus for generating an interpolation signal having a resolution higher than the period of a periodic signal and a position detecting apparatus using the same, and more particularly to a signal interpolation apparatus using sine and cosine wave signals generated in accordance with the displacement by an encoder or the like to obtain a resolution higher than the period thereof, in other words, shorter than the fundamental period. This apparatus can be used in the fields of precise position measurement and precise position control.
2. Related Background Art
An example of the conventional art is shown in FIG. 1 of the accompanying drawings. In FIG. 1, sine and cosine wave signals generated in accordance with displacement by an encoder or the like are converted into a rectangular wave having a frequency higher than the fundamental frequency of the sine wave signal by the use of a phase digitizer 21 for quantizing the sine and cosine wave signals in accordance with the phase angles thereof. From this signal, a count up pulse and a count down pulse are produced by an up/down pulse output circuit 22, and these are integrated by an up/down counter circuit 23, whereby a positional signal is outputted. As for the phase digitizer 21, use can be made of one having a construction as shown, for example, in FIG. 2 of the accompanying drawings.
As another example of the prior art, the construction described in Japanese Laid-Open Patent Application No. 54-19773 is shown in FIG. 3 of the accompanying drawings. In FIG. 3, sine and cosine signals are converted into digital values by an A/D converter 31 and input into a microprocessor 34, wherein the ratio between the values is found and the inverse tangent value thereof is calculated, and an interpolation process for finding the phase angle of the sine wave is carried out. Also, in parallel with this process, the sine and cosine signals are compared with 0 V by a comparator 32 and are thereby converted into a rectangular wave. Thereafter as in FIG. 1, a count up pulse and a count down pulse are produced by an up/down pulse output circuit 33. These are integrated by an up/down counter circuit 35, and the fundamental frequency of the sine wave signal is counted, which is thereby synthesized by a microprocessor 34 to thereby obtain positional information.
In the conventional art shown in FIG. 1, when the interpolation number is great and highly accurate position detection is required the circuit scale of the phase digitizer increases in proportion to the interpolation number. For example, in the construction of FIG. 2, the same number of comparators as the interpolation number becomes necessary and particularly, it is not realistic to digitize to 1/100 or smaller. Even if this problem is solved, the maximum response frequency of the up/down counter in the post stage of the phase digitizer is limited and therefore, it becomes impossible to detect highly accurately the position of an object which is moving at a high speed.
The example of the prior art shown in FIG. 3 intends to solve the above-noted problem to a certain degree, and the interpolation portion and the fundamental frequency counting portion are separated from each other to provide parallel construction, whereby even if the interpolation number is great, the circuit construction can be made small and moreover, the position of an object moving at a high speed can be detected highly accurately.
In the construction of FIG. 3, however, the A/D converting portion and the phase digitizing portion operate in parallel and therefore, their operations must be executed completely in synchronism with each other, and even when only slight shift in time has occurred. There is a possibility that mismatch occurs between positional information obtained from each portion. In such case, there is the possibility that positional information momentarily having a great error is outputted. For example, in a case where a servo system is constructed to use the detected positional information, and a momentarily great disturbance is applied to the servo system, there is a possibility that a fatal problem such as the collision of a moving object. Further, when the sine wave signal has noise, a similar problem arises and it has been impossible to obviate this.
Also, it is ideal that the sine and cosine wave signals obtained from the encoder or the like have equal amplitudes, but their amplitudes often fluctuate depending on position. Particularly when highly accurate detection is necessary, such amplitude fluctuation makes accurate interpolation calculation impossible, for example, in a portion for finding the inverse tangent. Therefore in the aforedescribed example, there is proposed a method of correcting this in a software-like fashion by the microprocessor 34. However, amplitude correction generally involves multiplying and dividing processes and therefore, the load of the microprocessor is high. If the same processor is used for the control calculation of the servo system, this becomes high load overhead and it becomes impossible to secure a short control period. It is possible to prepare a look-up table or the like and curtail the amount of multiplication and division, but in such case, the greater the interpolation number is, the the larger the look-up table, and it becomes difficult to store the table in memory effectively.